1. Field of the Invention
The invention generally relates to the area of mass spectroscopic analysis, and in particular to a multi reflecting time-of-flight mass spectrometer (MR-TOF MS) and a method of use.
2. State of the Art
Mass spectrometry is a well recognized tool of analytical chemistry, used for identification and quantitative analysis of various compounds and mixtures. The sensitivity and resolution of such analysis is an important concern for practical use. It has been well recognized that resolution of TOF MS is proportional to the length of the flight path. However, it is recognized it is difficult to increase the flight path while keeping the instrument to a reasonable size. A proposed solution is multi-reflecting time-of-flight mass spectrometers (M-TOF MS). The use of MR-TOF MS became possible after the introduction of an electrostatic ion mirror with time-of-flight focusing properties. U.S. Pat. No. 4,072,862, Soviet Patent No. SU198034 and Sov. J. Tech. Phys. 41 (1971) 1498 disclose an ion mirror to improve the focusing of ion energy in time-of-flight instruments. The use of the ion mirror automatically causes a single folding of ion flight path.
H. Wollnik realized a potential of ion mirrors for implementing a multi-reflecting MR-TOF MS. United Kingdom Patent No. GB2080021 suggests a way of reducing the full length of an instrument by folding the ion path between multiple gridless mirrors. Two rows of such mirrors may be aligned in the same plane or located on two opposite parallel circles (FIG. 1). Introduction of gridless ion mirrors with spatial ion focusing was intended to reduce ion losses and keep the ion beam confined regardless of the number of reflections (more details in U.S. Pat. No. 5,017,780). The gridless mirrors disclosed in GB 2080021 were to provide independence of ion flight time from the ion energy. Two types of MR-TOF MS are disclosed: (a) folded path’ scheme, which is equivalent to combining N sequential reflecting TOF MS, and where the flight path is folded along a jig-saw trajectory; and (b) ‘coaxial reflecting’ scheme, which employs multiple ion reflections between two axially aligned ion mirrors using pulsed ion admission and release. The ‘coaxial reflecting’ scheme was also described by H. Wollnik et al. in Mass Spec. Rev., 1993, 12, p.109 and was implemented in the work published in the Int. J. Mass Spectrom. Ion Proc. 227 (2003) 217. Resolution of 50,000 was achieved after 50 turns in a moderate size (30 cm) TOF MS. Gridless and spatially focusing ion mirrors indeed preserved ions of interest (losses were below factor of 2), though the admitted mass range shrank proportionally with the number of cycles.
Another type, cyclic MR-TOF MS was described in papers by H. Wollnik, Nucl. Instr. Meth., A258 (1987) 289, and Sakurai et al, Nucl. Instr. Meth., A427 (1999) 182. Ions are kept in closed orbits using electrostatic or magnetic deflectors. The scheme employed multiple repetitive cycles, which shrank mass range, similarly to the coaxial reflecting scheme.
A folded path MR-TOF MS using two-dimensional gridless mirrors was disclosed in Soviet Union Patent SU1725289. The MR-TOF MS comprised two identical mirrors, built of bars, were parallel and symmetric with respect to the median plane between the mirrors and also to the plane of the folded ion path (FIG. 2). Mirror geometry and potentials were arranged to focus the ion beam spatially across the plane of the folded ion path and provide second-order time of flight focusing with respect to the ion energy. The ions experienced multiple reflections between the planar mirrors, while slowly drifting towards the detector in a so-called shift direction (here X-axis). The number of cycles and resolution were adjusted by varying the ion injection angle.
Nazarenko's prototype of a ‘folded path’ MR-TOF MS with planar gridless mirrors, having spatial and time-of-flight focusing properties did not provide ion focusing in the shift direction, thus limiting the number of reflection cycles. Besides, the ion mirrors used in the prototype did not provide time-of-flight focusing with respect to spatial ion spread across the plane of the folded ion path, so that a use of diverging or wide beams would in fact ruin the time-of-flight resolution and would make an extension of flight path pointless. In other words, the scheme failed to deliver an acceptable analyzer and thus the ability of working with real ion sources. Lastly, the Nazarenko prototype has no implication on the type of ion source, nor on efficient ways of coupling between MR-TOF MS and various ion sources,
The type of ion source, its spatial and timing characteristics of ion beam, as well as geometrical constrains are the important considerations in the design of MR-TOF MS. Compatibility with single reflecting TOF MS does not automatically mean that a source is well suited for MR-TOF MS. For example, pulsed ion sources, like secondary ion SIMS or matrix-assisted desorption/ionization MALDI, are very compatible with TOF MS and such instruments are characterized by high resolution and moderate ion losses caused by spatial ion divergence. Switching to MR-TOF MS introduces new problems. On one hand, a pulsed nature of such sources suits well an extension of flight time in MR-TOF MS since frequency of ionizing pulses is adjustable. On the other hand, instability of MALDI ions is a limiting factor on flight time extension.
Gaseous ion sources, like electrospray (ESI), atmospheric pressure chemical ionization (APCI) atmospheric pressure photo-ionization (APPI), electron impact (EI), chemical ionization (CI), photo-ionization (CI) or inductively-coupled plasma (ICP) are known to produce stable ions, but they generate intrinsically continuous ion beams, or quasi-continuous ion beams, as in case of recently introduced gas filled MALDI ion source described in U.S. Pat. Nos. 6,331,702, and 6,504,150. TOF MS has been successfully coupled with continuous, and later to quasi-continuous ion sources, after introduction of an orthogonal ion acceleration scheme (o-TOF MS) (see U.S. Pat. No. 5,070,240, WO9103071, Soviet patent SU1681340), efficiently converting continuous ion beams into ion pulsed packets. Gaseous ion sources in combination with a collisional-cooling ion guide (U.S. Pat. No. 4,963,736) produce cold ion beams with low velocity spread along the axis of TOF MS, which help to achieve high TOF resolution in excess of 10,000. However, using MR-TOF MS would reduce the duty cycle of orthogonal acceleration and thus drop sensitivity.
U.S. Pat. No. 6,107,625 suggests that a further increase of resolution of o-TOF MS is mostly limited by a so-called ‘turn-around time’ and increasing of flight path improves resolution. The '625 patent suggests a coupling of external ESI source to a ‘coaxial reflecting’ MR-TOF MS via an orthogonal accelerator, combined with an ion mirror and multiple deflectors, such as shown in FIG. 3. To improve the sampling of the continuous ion beam, the interface employs a linear ion trap, storing ions between rare ion pulses. Melvin Park et. al. in the article entitled ‘Analytical Figure of Merits of a Multi-Pass Time-of-Flight Mass Spectrometer’, extended abstract on ASMS 2001, www.asms.org, MR-TOF MS demonstrated resolution of 60,000 using 6 cycles of reflections in a c.a. 1 m long instrument. However, the use of ion mirrors with grids caused severe ion scattering and ion losses. Coaxial reflecting MR-TOF MS improved resolution but shrank mass range proportionally.
ESI with orthogonal injection has been also coupled to an MR-TOF MS with a folded ion path (see EP 1 237 044 A2 and J. Hoyes et al. in extended abstract ASMS 2000 ‘A high resolution Orthogonal TOF with selectable drift length’ www.asms.org). The invention allows converting an existing commercial o-TOF into a dual reflecting instrument by introducing an additional short reflector between orthogonal source and detector. Energy of continuous ion beam controls number of ion reflections. The ‘folded path’ MR-TOF MS retains full mass range and considerably improves resolution, but it also reduces duty cycle and geometrical efficiency of ion sampling into the orthogonal accelerator in addition to ion losses and scattering occurring at every pass through meshes in both ion mirrors.
The two above examples demonstrate that a conventional orthogonal acceleration becomes inefficient in MR-TOF MS, particularly at extended flight times. There have been multiple attempts of improving pulsed ion sampling from continuous ion beams, mostly employing ion storage in radio-frequency (RF) traps, like 3-D ion trap (IT) in the paper of B. M. Chien et al. ‘The design and performance of an ion trap storage-reflectron time-of-flight mass spectrometer’ International Journal of Mass Spectrometry and Ion Processes 131 (1994) 149-119, linear ion trap (LIT) in U.S. Pat. No. 5,763,878, U.S. Pat. No. 5,847,386 (FIGS. 29-31), U.S. Pat. No. 6,111,250 (FIGS. 29-31), U.S. Pat. No. 6,545,268 and WO9930350 or dual LIT (GB2378312) and ring ion trap in paper of A. Luca et al., ‘On the combination of a linear field free trap with a time-of-flight mass spectrometer’, Rev. Sci. Instrum. V.72, #7 (2001), p 2900-2908. Since all of those solutions compromise temporal and/or spatial spread of ejected ion packets, the orthogonal injection is still the method of choice for singly reflecting TOF MS. Some trapping features are used in an intermediate scheme in U.S. Pat. No. 6,020,586, combining both an ion trapping step and an orthogonal acceleration. Slow ion packets are periodically ejected out of storing ion guide into a synchronized orthogonal accelerator. Compared to conventional o-TOF MS the scheme improves sensitivity, while moderately sacrificing resolution and mass range. The scheme has been coupled to coaxial MR-TOF MS in already described reference by M. Park. However, such instrument does not provide full mass range. It is still desirable to improve conversion of continuous ion beam into ion pulses fully suitable for TOF MS and particularly to multi-reflecting TOF MS.
Multiple reflecting TOF is also employed in tandem mass spectrometer in a co-pending application of one of the author (WO2004008481). A slow MR-TOF MS is used for slow separation of parent ions at a millisecond time scale and a short orthogonal TOF is used for fast mass analysis of fragments at a microsecond time scale. Fast collisional cell is used in-between to fragment ions without smearing time-of-flight separation in the MR-TOF MS. The scheme delivers a novel quality: it allows parallel or ‘multi-dimensional’ MS-MS analysis, where fragment spectra are simultaneously acquired for multiple parents without mixing them. The scheme has a drawback that parent ions spread in the shift direction which strongly limits acceptance of analyzer and requires smaller divergence of ion beam coming out of the ion source. A higher acceptance of MR-TOF MS is desirable.
Summarizing the above, the MR-TOF MS of the prior art do not have spatial and time of-flight focusing to provide a certain retaining of ion beam along a substantially extended flight path. Most of references describe MR-TOF analyzer without considering their compatibility with ion sources as well as their utility in tandem mass spectrometers. In fact, a limited acceptance of the known MR-TOF analyzers seriously limits such coupling and is expected to cause ion losses at substantially elongated flight paths. Some references are made to actual coupling of MR-TOF MS to continuous ion sources, demonstrating strong improvement of resolution. However, resolution is gained at the expense of losing sensitivity and, in the case of coaxial reflections, of shrinking mass range. Therefore, there is a need for TOF mass spectrometer working with intrinsically continuous or quasi-continuous ion sources, and superior to o-TOF by a set of major analytical characteristics, namely—sensitivity, mass range and resolution. There is also a need for better schemes of coupling TOF MS into tandem mass spectrometers.